ruby modify object

Question

I have code:

def crop_word (film_title)
  size = film_title.size
  film_title[0...size-2] if size > 4
end

film = "Electrocity"
p crop_word film

What must I do if I want to modify the object film? (How can I create crop_word method as a mutator method?)

p crop_word film #=> "Electroci"
p crop_word film #=> "Electro"
p crop_word film #=> "Elect"

Consider calling your method `crop_word!` to indicate it is mutative. — Marc-André Lafortune, Mar 24 '11 at 14:38

steenslag · Accepted Answer · 2011-03-24T15:03:10.490

4

def crop_word! (film_title)
  film_title.size > 4 ? film_title.slice!(0..-3) : film_title
end

puts crop_word! "1234567" #=>"12345"

edited Mar 24 '11 at 15:03

answered Mar 24 '11 at 14:08

steenslag

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Is it supposed to return `nil` `unless size > 4`? – sawa Mar 24 '11 at 14:49

score 1 · Answer 2 · edited May 23 '17 at 12:26

In Ruby you can't pass parameters by reference like in C-like languages. The easiest way is to return the new value and then assign in to the input variable.

film_title = crop_word(film_title)

What you can do is to place the film_title in a container.

class Film
  attr_accessor :title, :length
end

film = Film.new
film.title = "Butch Cassidy and the Sundance Kid"

def crop_word (film)
  length = film.title.length
  film.title=film.title[0..length-2] if length > 4
end

puts crop_word(film)
# Butch Cassidy and the Sundance K
puts crop_word(film)
# Butch Cassidy and the Sundance
puts crop_word(film)
# Butch Cassidy and the Sundan

I wouldn't recommend it but you could also monkey patch the String class

class String
  def crop_word!
    self.replace self[0..self.length-2] if self.length > 4
  end
end

title = "Fear and Loathing in Las Vegas"

title.crop_word!
# => "Fear and Loathing in Las Vega"
title.crop_word!
# => "Fear and Loathing in Las Veg"
title.crop_word!
# => "Fear and Loathing in Las Ve"

Finally there's the black magic of eval and binding which you probably would have to be insane to actually use.

def crop_word(s, bdg)
  eval "#{s}.chop!.chop! if #{s}.length > 4", bdg
end

title="The Dark Knight"
crop_word(:title, binding)
puts title
# The Dark Knig
crop_word(:title, binding)
puts title
# The Dark Kn
crop_word(:title, binding)
puts title
# The Dark

Also, your crop_word does not output what you seem to want since it keeps the trailing spaces.

Sorry, that's just wrong. There's no need for a wrapper here, the String#slice! method will do the job just fine. — regularfry, Mar 24 '11 at 16:12
Of course, this wasn't meant to be a solution of string slicing but meant as a wider explanation on what Ruby has instead of reference/output variables. — Jonas Elfström, Mar 24 '11 at 18:49

score 0 · Answer 3 · answered Mar 24 '11 at 12:43

0

def crop_word (film_title)
  size = film_title.size
  film_title[size-2..size]="" if size > 4
  film_title
end

In general you either have to use a method that already does an in-place mutation or reopen the relevant class and assign to self.

answered Mar 24 '11 at 12:43

Jakub Hampl

39,863
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score 0 · Answer 4 · answered Mar 24 '11 at 14:43

0

The question is not clear. I suppose you want to remove the last character if it is longer than 4.

class String
    def crop_word!; replace(self[0..(length > 4 ? -2 : -1)]) end
end


puts 'Electrocity'.crop_word! # => 'Electrocit'

answered Mar 24 '11 at 14:43

sawa

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ruby modify object

4 Answers4